Surface acoustic wave device

ABSTRACT

The invention provides a surface acoustic wave device which has a thin film formed on a surface of a substrate adapted to excite longitudinal wave-type surface acoustic waves, longitudinal wave-type quasi surface acoustic waves or longitudinal wave-type surface skimming bulk waves to thereby give an increased electromechanical coupling coefficient and at the same time minimize the temperature coefficient of delay time. For example, in a surface acoustic wave device having an aluminum thin film formed on a surface of a lithium tantalate substrate, the direction of propagation of longitudinal wave-type quasi surface acoustic waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg) as expressed in Eulerian angles and within a range equivalent thereto, and the product of wave number of longitudinal wave-type quasi surface acoustic waves and the thickness of the thin film is at least 1.0, preferably in the range of 1.3 to 2.0. The device provided exhibits higher performance than in the paior art.

CONTINUING DATA

This application is a division of Ser. No. 08/629,403 filed Apr. 9,1996, now U.S. Pat. No. 5,760,522 issued Jun. 2, 1998.

FIELD OF THE INVENTION

The present invention relates to surface acoustic wave devicescomprising a substrate adapted to excite surface acoustic waves whereinthe longitudinal wave component predominates over the shear wavecomponent, quasi surface acoustic waves wherein the longitudinal wavecomponent predominates over the shear wave component, or surfaceskimming bulk waves wherein the longitudinal wave component predominatesover the shear wave component.

BACKGROUND OF THE INVENTION

In recent years, surface acoustic wave devices have found wideapplication in communication devices such as a motor vehicle telephonesas circuit elements of resonator filters, signal processing delay lines,etc. For example, FIG. 13 shows a surface acoustic wave devicecomprising interdigital electrodes 2 and latticelike reflectors 3, 3which are formed on the surface of a piezoelectric substrate 1. Thedevice converts electric signals to surface acoustic waves and viceversa.

Surface acoustic waves are surface waves which literally propagate alongthe surface of an elastic body, and the energy thereof is not radiatedinto the substrate. A plurality of modes of excitation have beendiscovered as such surface acoustic waves. For example, already knownare the Rayleigh wave, Sezawa wave, Love wave, electroacoustic wave,etc.

In the Rayleigh and Sezawa waves, predominant are both a longitudinalwave component having a displacement in the same direction as thedirection of propagation and a shear wave component having adisplacement depthwise of the substrate. On the other hand, predominantin the Love wave and the electroacoustic wave is a shear wave componenthaving a displacement in parallel to the substrate surface andperpendicular to the propagation direction. While three kinds of bulkwaves, i.e., "slow shear wave", "fast shear wave" and "longitudinalwave" are present in the piezoelectric substrate, the surface acousticwaves propagate at a phase velocity lower than that of the "slow shearwave".

Also known are elastic waves which propagate along the surface of anelastic body while radiating energy depthwise of the body. These wavesare called quasi surface acoustic waves or leaky surface acoustic waves.The quasi surface acoustic wave initially discovered comprises apredominant shear wave component having a displacement in parallel tothe substrate surface and perpendicular to the propagation direction,and is intermediate between the "slow shear wave" and the "fast shearwave" in phase velocity.

Quasi surface acoustic waves having a predominant longitudinal wavecomponent are discovered in recent years one after another (seeJP-A-112763/1994 and proceedings of the 15th Symposium on Fundamentalsand Applications of Ultrasonic Electronics, 1994, pp. 185-186). Thesequasi surface acoustic waves having a predominant longitudinal wavecomponent are intermediate between the "fast shear wave" and the"longitudinal wave in phase velocity".

On the other hand, there is a case wherein bulk waves propagating alongand close to the surface of a substrate are excited by interdigitalelectrodes and detected by other interdigital electrodes on the samesubstrate. Such bulk waves are termed surface skimming bulk waves. It isthought that there are three kinds of surface skimming bulk waves incorresponding relation with the usual bulk waves. However, mainlyhandled at present is the surface skimming bulk wave which comprises apredominant shear wave component having a displacement in parallel tothe substrate surface and perpendicular to the propagation direction.

The characteristics of elastic waves include acoustic velocity,propagation loss, temperature characteristics of delay time andelectromechanical coupling coefficient. These characteristics relatedirectly to the design parameters of the circuit to which the surfaceacoustic wave device is applied.

The period (center-to-center distance) of the electrode fingers ofinterdigital electrodes or latticelike reflectors has a value which is1/2 of the wavelength λ of elastic waves, so that at a constantfrequency, the smaller the acoustic velocity, the smaller is thewavelength and the more difficult are the electrodes to fabricate. It istherefore desired that the acoustic velocity be greater.

The resonance sharpness of surface acoustic wave resonators and theinsertion loss of surface acoustic wave filters are dependent directlyon the propagation loss of surface acoustic waves. For this reason, thepropagation loss should preferably be small.

The high-frequency devices for use in mobile communication are used at afrequency specified by the standard. Accordingly, it is not desirablethat the frequency varies with variations in temperature. Thetemperature coefficient of delay time should preferably be small,therefore.

The electromechanical coupling coefficient represents a capacity toconvert the energy of an input electric signal into the energy ofsurface acoustic waves. When the interdigital electrodes have asufficiently increased number of electrode fingers, elastic waves ofdesired energy can be excited even if the electromechanical couplingcoefficient is small, whereas the interdigital electrodes then have anincreased electrical capacity, which presents difficulty in impedancematching with an external circuit, necessitating an additional matchingcircuit for impedance matching. Further it is known that the number ofelectrode fingers of interdigital electrodes is approximately in inverseproportion to the operating frequency range of the surface acoustic wavedevice, such that an increase in the number of electrode fingers limitsthe realizable characteristics to a narrow frequency range. Accordingly,the electromechanical coupling coefficient is preferably great.

Already known are substrate conditions (e.g., the relationship betweenthe crystal axis and the direction of propagation of surface acousticwaves) and electrode conditions (e.g., the center-to center distance andfilm thickness of the electrode fingers) for improving the foregoingcharacteristics in connection with elastic waves (such as the Rayleighwave and Sezawa wave) which comprise two predominant components, i.e., ashear wave component having a depthwise displacement and a longitudinalwave component, and elastic waves (such as the electroacoustic wave,Love wave, quasi surface acoustic wave of the shear wave type andsurface skimming bulk wave of the shear wave type) which comprise apredominant shear wave component having a displacement in parallel tothe surface and perpendicular to the propagation direction (Proceedingsof the 1994 IEICE (Institute of Electronics, Information andCommunication Engineers) Spring Conference, "A-438", "A-437", "A-438",Japanese Journal of Applied Physics, vol. 29 (1990) Supplement 29-1, pp.119-121, Japanese Journal of Applied Physics, vol. 30 (1991) Supplement30-1, pp. 143-145, etc.).

However, the requirements of the substrate and electrodes for improvingthe above characteristics still remain to be clarified for surfaceacoustic waves wherein the longitudinal wave component predominates overthe shear wave component (longitudinal wave-type surface acousticwaves), quasi surface acoustic waves wherein the longitudinal wavecomponent predominates over the shear wave component (longitudinalwave-type quasi surface acoustic waves) and surface skimming bulk waveswherein the longitudinal wave component predominates over the shear wavecomponent (longitudinal wave-type surface skimming bulk waves).

SUMMARY OF THE INVENTION

An object of the present invention is to clarify the requirements of thesubstrate and electrodes for improving the elastic wave characteristicsof surface acoustic wave devices comprising a substrate which is adaptedto excite longitudinal wave-type surface acoustic waves, longitudinalwave-type quasi surface acoustic waves or longitudinal wave-type surfaceskimming bulk waves, and to provide surface acoustic wave devices whichare improved over conventional devices in performance.

The present invention provides a surface acoustic wave device whichcomprises a substrate adapted to excite longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and in which a thinfilm is formed on a surface of the substrate to thereby give the deviceimproved characteristics of propagating longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves.

More specifically, the thin film on the substrate is made of a metal,and exciting electrodes are formed by the thin film.

The invention provides a surface acoustic wave device which comprises asubstrate of lithium tantalate (LiTaO₃) and a thin film formed on asurface of the substrate and comprising aluminum or an alloy consistingmainly of aluminum, and in which the direction of propagation oflongitudinal wave-type surface acoustic waves, longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 1.0, preferably in the range of 1.3to 2.0.

The invention also provides a surface acoustic wave device whichcomprises a substrate of lithium tantalate and a thin film formed on asurface of the substrate and comprising gold or an alloy consistingmainly of gold, and in which the direction of propagation oflongitudinal wave-type surface acoustic waves, longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 0.05, preferably in the range of1.0 to 1.4.

The invention further provides a surface acoustic wave device whichcomprises a substrate of lithium niobate (LiNbO₃) and a thin film formedon a surface of the substrate and comprising aluminum or an alloyconsisting mainly of aluminum, and in which the direction of propagationof longitudinal wave-type surface acoustic waves, longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 0.3, preferably in the range of 1.0to 2.0.

The invention further provides a surface acoustic wave device whichcomprises a substrate of lithium niobate and a thin film formed on asurface of the substrate and comprising gold or an alloy consistingmainly of gold, and in which the direction of propagation oflongitudinal wave-type surface acoustic waves, longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 0.02, preferably in the range of0.8 to 2.0.

The invention further provides a surface acoustic wave device whichcomprises a substrate of lithium tetraborate (Li₂ B₄ O₇) and a thin filmformed on a surface of the substrate and comprising aluminum or an alloyconsisting mainly of aluminum, and in which the direction of propagationof longitudinal wave-type surface acoustic waves longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (0 deg to 50 deg, 15 deg to 75 deg, 40 deg to 90 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 0.3, preferably up to 1.9.

The invention further provides a surface acoustic wave device whichcomprises a substrate of lithium tetraborate and a thin film formed on asurface of the substrate and comprising gold or an alloy consistingmainly of gold, and in which the direction of propagation oflongitudinal wave-type surface acoustic waves, longitudinal wave-typequasi surface acoustic waves or longitudinal wave-type surface skimmingbulk waves is (0 deg to 50 deg, 15 deg to 75 deg, 40 deg to 90 deg) asexpressed in Eulerian angles and within a range equivalent thereto, theproduct of the wave number (1/μm) of longitudinal wave-type surfaceacoustic waves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves and the thickness(μm) of the thin film being at least 0.02, preferably up to 0.15 or inthe range of 0.4 to 2.0.

Longitudinal wave-type quasi surface acoustic waves have a major portionof their energy concentrated on a range of from the surface to a depthcorresponding to several wavelengths. Accordingly, a thin film, whenformed on the substrate, influences the characteristics of the elasticwaves. For example, if the acoustic velocity of the thin film is lowerthan that of the substrate, the phase velocity of the longitudinalwave-type quasi surface acoustic waves decreases.

With interdigital electrodes and latticelike reflectors, a region ofmetal thin film providing an electrode finger alternates with a regionof the other free surface. When the difference in phase velocity betweenthese two regions increases, the electromechanical coupling coefficientalso increases. Further the metal thin film has two effect to reduce thephase velocity, i.e., electrical short-circuiting effect and masseffect. The latter effect is dependent on the thickness of the thin filmand the density of the metal.

Accordingly, the electromechanical coupling coefficient can be alteredby changing the electrode-forming metal and varying the thickness of theelectrode film.

The phase velocity of elastic waves further depends on temperature sincethe dielectric constant, piezoelectric constant, elastic constant anddensity of the piezoelectric substrate vary with temperature. Theelastic constant and density of the thin film also vary withtemperature. When the thin film is prepared from a suitable materialwith a suitable thickness, the temperature characteristics of thesubstrate material offset those of the thin film material, greatlydecreasing the temperature coefficient of the surface acoustic wavedevice.

It is known that longitudinal wave-type quasi surface acoustic waves areusually faster than the "fast shear wave" and slower than the"longitudinal wave", whereas the slow shear wave component and the "fastshear wave" component of the quasi surface acoustic waves are unable topropagate in this phase velocity range, and are radiated into thesubstrate. However, if a thin film with a sufficiently low acousticvelocity and a thickness of not smaller than a certain value is formedon the surface of the piezoelectric substrate, the velocity of the quasisurface acoustic waves becomes lower than that of the fast shear wave .In this case, the "rapid shear" wave component is no longer radiatedinto the substrate, hence a reduced propagation loss. When the thicknessis increased, the velocity decreases below that of the "slow shearwave", with the result that the longitudinal wave-type quasi acousticwaves become longitudinal wave-type surface acoustic waves. With bulkwaves no longer radiated into the substrate, the propagation losstheoretically diminishes to zero.

The characteristics of longitudinal wave-type surface acoustic waves andlongitudinal wave-type surface skimming bulk waves can be improved alsoby the same effects as described above for the longitudinal wave-typequasi surface acoustic waves.

In the case where longitudinal wave-type quasi surface acoustic wavesand longitudinal wave-type surface skimming bulk waves appear under thesame conditions on attenuation, the same effects as above are of courseavailable.

With the surface acoustic wave device of the invention having asubstrate adapted to excite longitudinal wave-type surface acousticwaves, longitudinal wave-type quasi surface acoustic waves orlongitudinal wave-type surface skimming bulk waves, a thin film ofsuitable material and thickness is formed on a surface of the substrate,thereby providing an increased electromechanical coupling coefficientfor surface acoustic waves, quasi surface acoustic waves or surfaceskimming bulk waves of the longitudinal wave type and minimizing thetemperature coefficient of delay time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising an aluminum thin film formed ona lithium tantalate substrate.

FIG. 2 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 3 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising a gold thin film formed on alithium tantalate substrate.

FIG. 4 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 5 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising an aluminum thin film formed ona lithium niobate substrate.

FIG. 6 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 7 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising a gold thin film formed on alithium niobate substrate.

FIG. 8 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 9 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising an aluminum thin film formed ona lithium tetraborate substrate.

FIG. 10 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 11 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and theelectromechanical coupling coefficient thereof as established for asurface acoustic wave device comprising a gold thin film formed on alithium tetraborate substrate.

FIG. 12 is a graph showing the relationship between the KH parameter oflongitudinal wave-type quasi surface acoustic waves and the temperaturecoefficient of delay time as established for the surface acoustic wavedevice.

FIG. 13 is a diagram showing the construction of electrodes of a surfaceacoustic wave device.

DETAILED DESCRIPTION OF EMBODIMENTS

In connection with embodiments which comprise a substrate adapted toexcite longitudinal wave-type quasi surface acoustic waves and made oflithium tantalate, lithium niobate or lithium tetraborate, andelectrodes of aluminum or gold formed on the substrate, the requirementsof the substrate and requirements of the electrodes will be clarifiedbelow for realizing an increased electromechanical coupling coefficientand a minimized temperature coefficient of delay time.

The characteristics of surface acoustic wave devices were evaluated byusing a common analysis method already known (see, for example, J. J.Campbell, W. R. Jones, "A Method for Estimating Optimal Crystal Cuts andPropagation Directions for Excitation of Piezoelectric Surface Waves",IEEE Transaction on Sonics and Ultrasonics, vol, SU-15, No. 4, pp.209-217, (1968)), constructing models of surface acoustic wave deviceshaving a thin film over the entire surface area of the substrate, andcalculating electromechanical coupling coefficients and temperaturecoefficients by computer simulation.

FIG. 1 shows characteristics of a surface acoustic wave devicecomprising an aluminum thin film on a lithium tantalate substrate byplotting the wave number K of longitudinal wave-type quasi surfaceacoustic waves (K=2 π/λ) multiplied by the thickness H (μm) of the thinfilm ,i.e., the product KH (hereinafter referred to as KH parameter), asabscissa and the electromechanical coupling coefficient of longitudinalwave-type quasi surface acoustic waves as ordinate.

The direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg),preferably (80 deg to 90 deg, 80 deg to 90 deg, 20 deg to 40 deg), morepreferably (88 deg to 90 deg, 88 deg to 90 deg, 30 deg to 32 deg), mostpreferably (90 deg, 90 deg, 31 deg), as expressed in Eulerian angles.The superiority of these angle ranges is experimentally substantiated.

As apparent from FIG. 1, when the KH parameter is over about 1.0, theelectromechanical coupling coefficient is at least 10% as desired fordesign. When the KH parameter is over about 1.3, the electromechanicalcoupling coefficient is at least 20%, which is more desirable. When theKH parameter is over about 1.7, the coupling coefficient is at least50%, which is most desirable.

FIG. 2 shows characteristics of the above surface acoustic wave device,with the KH parameter plotted as abscissa and with the temperaturecoefficient of delay time as ordinate. When the KH parameter is in therange of about 1.3 to about 2.0, the temperature coefficient of delaytime is up to 20 ppm/° C. as desired for design. With the KH parameterin the range of about 1.4 to about 1.8, the temperature coefficient ofdelay time is up to 10 ppm/° C., which is more desirable. When the KHparameter is about 1.6, the temperature coefficient is 5 ppm/° C., whichis most desirable.

In FIG. 3, the KH parameter is plotted as abscissa, and theelectromechanical coupling coefficient as ordinate, to showcharacteristics of a surface acoustic wave device comprising a gold thinfilm formed on a lithium tantalate substrate.

the direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg)preferably (80 deg to 90 deg, 80 deg to 90 deg, 20 deg to 40 deg), morepreferably (88 deg to 90 deg, 88 deg to 90 deg, 30 deg to 32 deg), mostpreferably (90 deg, 90 deg, 31 deg), as expressed in Eulerian angles.The superiority of these angular ranges is experimentally substantiated.

As apparent from FIG. 3, when the KH parameter is over about 0.05, theelectromechanical coupling coefficient is at least 10% as desired fordesign. When the KH parameter is over about 0.10, the electromechanicalcoupling coefficient is at least 20%, which is more desirable. When theKH parameter is over about 0.2, the coupling coefficient is at least50%, which is most desirable.

FIG. 4 shows characteristics of the above surface acoustic wave device,with the KH parameter plotted as abscissa and the temperaturecoefficient of delay time as ordinate. When the KH parameter is about1.0 to about 1.4, the absolute value of the temperature coefficient ofdelay time is up to 20 ppm/° C. as desired for design. When the KHparameter is about 1.3 to about 1.4, the absolute value of thetemperature coefficient of delay time is up to 10 ppm/° C., which ismore desirable. When the product of the thickness of the gold film andthe wave number of longitudinal wave-type quasi surface acoustic wavesis about 1.3, the temperature characteristic of delay time isapproximately 0 ppm/° C., which is most desirable.

If the KH parameter is at least 0.6, the phase velocity becomes lowerthan those of the fast shear wave and the "slow shear wave", so that inactuality the longitudinal wave-type quasi surface acoustic waves arenot such but behave as longitudinal wave-type surface acoustic waves.Needless to say, therefore, the propagation loss is nearly zero.

FIG. 5 wherein the KH parameter is plotted as abscissa, andthe-electromechanical coupling coefficient of longitudinal wave-typequasi surface acoustic waves as ordinate shows characteristics of asurface acoustic wave device comprising an aluminum thin film formed ona lithium niobate substrate.

The direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg),preferably (80 deg to 90 deg, 80 deg to 90 deg, 20 deg to 40 deg), morepreferably (88 deg to 90 deg, 88 deg to 90 deg, 36 deg to 38 deg), mostpreferably (90 deg, 90 deg, 37 deg, as expressed in Eulerian angles. Thesuperiority of these angular ranges is experimentally substantiated.

As will be apparent from FIG. 5, when the KH parameter is at least about0.3, the electromechanical coupling coefficient is in excess of 20% asdesired for design. When the KH parameter is at least about 0.8, theelectromechanical coupling coefficient has a more desirable value of atleast 30%.

FIG. 6 wherein the KH parameter is plotted as abscissa, and thetemperature coefficient of delay time as ordinate shows characteristicsof the surface acoustic wave device. When the KH parameter is about 1.0to about 2.0, the temperature coefficient of delay time is up to 40ppm/° C. as desired for design. When the KH parameter is about 1.0 toabout 1.2, the temperature coefficient of delay time is up to 30 ppm/°C., which is more desirable. When the KH parameter is about 1.1, thetemperature coefficient has the most desirable value of 20 ppm/° C.

FIG. 7, wherein the KH parameter is plotted as abscissa, and theelectromechanical coupling coefficient of longitudinal wave-type quasisurface acoustic waves as ordinate, chows characteristics of a surfaceacoustic wave device comprising a gold thin film formed on a lithiumniobate substrate.

The direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60 deg),preferably (80 deg to 90 deg, 80 deg to 90 deg, 20 deg to 40 deg), morepreferably (88 deg to 90 deg, 88 deg to 90 deg, 36 deg to 38 deg), mostpreferably (90 deg, 90 deg, 37 deg), as expressed in Eulerian angles.The superiority of these angular ranges is experimentally substantiated.

As will be apparent from FIG. 7, when the KH parameter is at least about0.02, the electromechanical coupling coefficient is at least 20% asdesired for design. When the KH parameter is at least about 0.05, thecoupling coefficient has a more desirable value of at least 30%.

FIG. 8 wherein the KH parameter is plotted as abscissa, and thetemperature coefficient of delay time as ordinate shows characteristicsof the surface acoustic wave device. When the KH parameter is about 0.8to about 2.0, the absolute value of the temperature coefficient of delaytime is up to 20 ppm/° C. as desired for design. When the KH parameteris about 1.0 to about 2.0, the temperature coefficient of delay time hasa more desirable value of up to 10 ppm/° C. When the KH parameter, i.e.,the product of the thickness of the gold film and the wave number oflongitudinal wave-type quasi surface acoustic waves, is about 1.2 to1.6, the temperature coefficient is nearly 0 ppm/° C. which is mostdesirable.

If the KH parameter is at least 0.3, the phase velocity becomes lowerthan those of the "fast shear wave" and the "slow shear wave", so thatin actuality the longitudinal wave-type quasi surface acoustic waves arenot such but behave as longitudinal wave-type surface acoustic waves.Needless to say, therefore, the propagation loss is then nearly zero.

FIG. 9 wherein the KH parameter is plotted as abscissa, and theelectromechanical coupling coefficient as ordinate shows characteristicsof a surface acoustic wave device comprising an aluminum thin filmformed on a lithium tetraborate substrate.

The direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (0 deg to 50 deg, 15 deg to 75 deg, 40 deg to 90 deg),preferably (0 deg to 10 deg, 40 deg to 50 deg, 80 deg to 90 deg) ,morepreferably (0 deg to 2 deg, 44 deg to 46 deg, 88 deg to 90 deg), mostpreferably (0 deg, 45 deg, 90 deg), as expressed in Eulerian angles. Thesuperiority of these angular ranges is experimentally substantiated.

As will be apparent from FIG. 9, when the KH parameter is at least about0.3, the electromechanical coupling coefficient is at least 10% asdesired for design. When the KH parameter is at least 0.6, theelectromechanical coupling coefficient has a more desirable value of atleast 20%. When the KH parameter is about 1.6, the coupling coefficienthas the most desirable value of about 50%.

FIG. 10 shows characteristics of the surface acoustic wave device, withthe KH parameter plotted as abscissa, and the temperature coefficient ofdelay time as ordinate. When the KH parameter is up to about 1.9, theabsolute value of the temperature coefficient of delay time is up to 20ppm/° C. as desired for design. When the KH parameter is up to about0.5, the temperature coefficient of delay time has a more desirablevalue of up to 5 ppm/° C. When the KH parameter is about 0.1, about 0.4,about 1.7 or about 1.95, the temperature coefficient is nearly 0 ppm/°C. which is most desirable.

When the KH parameter is set in the range of about 1.6 to about 1.95,both the electromechanical coupling coefficient and the temperaturecoefficient of delay time can be optimized.

FIG. 11 wherein the KH parameter is plotted as abscissa, and theelectromechanical coupling coefficient as ordinate chows characteristicsof a surface acoustic wave device comprising a gold thin film formed ona lithium tetraborate substrate.

The direction of propagation of longitudinal wave-type quasi surfaceacoustic waves is (0 deg to 50 deg, 15 deg to 75 deg, 40 deg to 90 deg),preferably (0 deg to 10 deg, 40 deg to 50 deg, 80 deg to 90 deg), morepreferably (0 deg to 2 deg, 44 deg to 46 deg, 88 deg to 90 deg), mostpreferably (0 deg, 45 deg, 90 deg), as expressed in Eulerian angles. Thesuperiority of these angular ranges is experimentally substantiated.

As will be apparent from FIG. 11, when the KH parameter is at leastabout 0.02, the electromechanical coupling coefficient is at least 10%as desired for design. When the KH parameter is at least about 0.05, theelectromechanical coupling coefficient has a more desirable value of atleast 20%. When the KH parameter is at least about 0.1, the couplingcoefficient has the most desirable value of at least 40%.

FIG. 12 shows characteristics of the surface acoustic wave device, withthe KH parameter plotted as abscissa and the temperature coefficient ofdelay time as ordinate. When the KH parameter is up to about 0.15 orabout 0.4 to about 2.0, the temperature coefficient of delay time is upto 20 ppm/° C. as desired for design. When the KH parameter is up toabout 0.1 or about 1.7 to about 2.0, the temperature coefficient ofdelay time has more desirable value of up to 10 ppm/° C. If the KHparameter is at least 0.2, the phase velocity is lower than that of the"fast shear wave" with no radiation of the "fast shear wave", hence agreatly diminished propagation loss. Further if the KH parameter is atleast 0.3, the phase velocity becomes lower than that of the "slow shearwave", with the result that the longitudinal wave-type quasi surfaceacoustic waves are no longer such but behave as longitudinal wave-typesurface acoustic waves. Needless to say, therefore, the propagation lossis then nearly zero.

In calculating the electromechanical coupling coefficients and thetemperature coefficients of delay time by the aforementioned computersimulation, models are used wherein a thin film is formed over theentire surface area of a substrate. Accordingly, in establishing therelationship between the KH parameter and the temperature coefficient ofdelay time shown in each of FIG. 2, FIG. 4, FIG. 6, FIG. 8 and FIG. 10for surface acoustic wave filters wherein interdigital electrodes areformed on the substrate surface, it is necessary to use the product ofthe wave number and the average film thickness of the electrode-formingregion. The average film thickness of the electrode-forming region iscalculated by multiplying the film thickness of the electrode by theduty ratio (width of the electrode finger/electrode period).

As described above, the surface acoustic wave device of the presentinvention comprises a substrate of lithium tantalate, lithium niobate orlithium tetraborate, and an aluminum or gold thin film of specifiedthickness formed on the substrate. This structure makes it possible toincrease the electromechanical coupling coefficient of longitudinalwave-type quasi surface acoustic waves and to minimize the temperaturecoefficient of delay time at the same time.

The embodiments described above are intended to illustrate the presentinvention and should not be construed as limiting the invention asdefined in the appended claims or reducing the scope thereof. Thedevices of the invention are not limited to the above embodiment inconstruction but can of course be modified variously without departingfrom the spirit of the invention set forth in the claims.

For example, surface acoustic wave devices which have a thin film formedon a substrate and prepared from a piezoelectric material different fromthat of the substrate, and electrodes formed on the thin film can beexpected to have the same advantage as above due to the mass effect ofthe thin film.

What is claimed is:
 1. A surface acoustic wave device comprising:asubstrate made of lithium niobate and adapted to excite surface acousticwaves in which a longitudinal wave component predominates over a shearwave component, quasi surface acoustic waves in which a longitudinalwave component predominates over a shear wave component, or surfaceskimming bulk waves in which a longitudinal wave component predominatesover a shear wave component; and a thin film comprising aluminum or analloy mainly comprising aluminum and being formed on a surface of thesubstrate, wherein the direction of the propagation of surface acousticwaves in which a longitudinal wave component predominates over a shearwave component, quasi surface acoustic waves in which a longitudinalwave component predominates over a shear wave component, or surfaceskimming bulk waves in which a longitudinal wave component predominatesover a shear wave component is (40 deg to 90 deg, 40 deg to 90 deg, 0deg to 60 deg) as expressed in Eulerian angles and within a rangeequivalent thereto, and the product of the wave number of the surfaceacoustic waves, quasi surface acoustic waves or surface skimming bulkwaves of the type mentioned and the thickness of the thin film is in therange of 1.0 to 2.0.
 2. A surface acoustic wave device comprising:asubstrate made of lithium niobate and adapted to excite surface acousticwaves in which a longitudinal wave component predominates over a shearwave component, quasi surface acoustic waves in which a longitudinalwave component predominates over a shear wave component, or surfaceskimming bulk waves in which a longitudinal wave component predominatesover a shear wave component; and a thin film comprising gold or an alloymainly comprising of gold and being formed on a surface of thesubstrate, wherein the direction of the propagation of surface acousticwaves in which a longitudinal wave component predominates over a shearwave component, quasi surface acoustic waves in which a longitudinalwave component predominates over a shear wave component, or surfaceskimming bulk waves in which a longitudinal wave component predominatesover a shear wave component is (40 deg to 90 deg, 40 deg to 90 deg, 0deg to 60 deg) as expressed in Eulerian angles and within a rangeequivalent thereto, and the product of the wave number of the surfaceacoustic waves, quasi surface acoustic waves or surface skimming bulkwaves of the type mentioned and the thickness of the thin film is in therange of 0.8 to 2.0.
 3. A surface acoustic wave device comprising asubstrate of lithium niobate and exciting electrodes formed on a surfaceof the substrate and comprising aluminum or an alloy consisting mainlyof aluminum, the surface acoustic wave device being characterized inthat the electrodes are so formed that the direction of propagation ofsurface acoustic waves in which a longitudinal wave componentpredominates over a shear wave component, quasi surface acoustic wavesin which a longitudinal wave component predominates over a shear wavecomponent, or surface skimming bulk waves in which a longitudinal wavecomponent predominates over a shear wave component is (40 deg to 90 deg,40 deg to 90 deg, 0 deg to 60 deg) as expressed in Eulerian angles andwithin a range equivalent thereto, and the product of the wave number ofthe surface acoustic waves, quasi surface acoustic waves or surfaceskimming bulk waves of the type mentioned and an average film thicknessof an electrode-forming region being in the range of 1.0 to 2.0.
 4. Asurface acoustic wave device comprising a substrate of lithium niobateand exciting electrodes formed on a surface of the substrate andcomprising gold or an alloy consisting mainly of gold, the surfaceacoustic wave device being characterized in that the electrodes are soformed that the direction of propagation of surface acoustic waves inwhich a longitudinal wave component predominates over a shear wavecomponent, quasi surface acoustic waves in which a longitudinal wavecomponent predominates over a shear wave component, or surface skimmingbulk waves in which a longitudinal wave component predominates over ashear wave component is (40 deg to 90 deg, 40 deg to 90 deg, 0 deg to 60deg) as expressed in Eulerian angles and within a range equivalentthereto, and the product of the wave number of the surface acousticwaves, quasi surface acoustic waves or surface skimming bulk waves ofthe type mentioned and an average film thickness of an electrode-formingregion being in the range of 0.8 to 2.0.